A heating-cooling control system includes thermostats in the discharge and return air passages, and a temperature set dial. A selection circuit energizes one of the thermostats below a preset temperature, and the other thermostat is energized above that temperature. A heat override circuit prevents a...http://www.google.fr/patents/US3768545?utm_source=gb-gplus-shareBrevet US3768545 - Temperature control system with multiple thermostats

A heating-cooling control system includes thermostats in the discharge and return air passages, and a temperature set dial. A selection circuit energizes one of the thermostats below a preset temperature, and the other thermostat is energized above that temperature. A heat override circuit prevents a call for heat above a certain temperature.

57 ABSTRACT [22] Filed: July 12, 1972 A heating-cooling control system includes thermostats [21] Appl No: 271,087 in the discharge and return air passages, and a temperature set dial. A selection clrcuit energizes one of the thermostats below a preset temperature, and the other [52] U.S.Cl 165/27, 236/91, 307/39 thermostat is energized above that temperature. A [51] Int. Cl..... F251) 29/00 heat override circuit prevents a call for heat above a [58] Field 01 Search 165/14, 22, 26, 27; certain temperature.

SIGNAL HLAT 32 HEATING OVERRIDE PATENTEDUU 30 I975. I 3.768545 SHEET 28F 3 I RETURN DISCHARGE l4 Wser W'W i W FIGZ ' SHEET 0F 3 TO HOT GAS CONTROL T0 2 as (2%: 8 I05 I06 I07 COM 97 95 9e HEAT CONTROL TEMPERATURE CONTROL SYSTEM WITH MULTIPLE THERMOSTATS BACKGROUND OF THE INVENTION Heating-cooling systems which operate over considerable load variations with multiple compressors have been developed for some time. There are still problems associated with such systems. One problem is with the cooling of fresh produce, such as lettuce, carried in railroad box cars. In general such systems utilize a single thermostat sensing temperature below the top of the produce mass, so that when this sensed temperature indicates suitable cooling of the load, the top layer of the produce is frequently frozen. This frozen material is wasted and must be discarded.

Another problem frequently encountered is the cycling on and off of closely controlled equipment as the sensed temperature varies up and down, just above and below the control point. In a multi-compressor cooling syste, it can happen that more than one compressor is brought on the line simultaneously by a high cooling demand, causing an overload of the electrical supply circuit or equipment.

It is a principal consideration of this invention to provide an improved heating-cooling system which obviates the freezing and destruction of the top layer of produce, when perishables are cooled by the system.

Another important consideration of the invention is to provide a multiple compressor system in which the first compressor energized is not cycled on and off, but delayed in its re-energization, to obviate excess cycling.

Still another important consideration of the invention is the provision of a system which regulates energization and de-energization of the first compressor, loading and unloading of the first compressor, energization and deenergization of a second compressor, and second compressor loading and unloading, in the appropriate sequence to minimize excessive drain in the electrical supply circuit.

SUMMARY OF THE INVENTION A control arrangement constructed in accordance with this invention is useful to regulate operation of a cooling system which discharges air into a space and receives return'air from that space. The control arrangement comprises a first thermostat disposed to provide a first temperature signal indicative of the discharge air temperature, and a second thermostat disposed to provide a second temperature signal indicative of the return air temperature. An adjustable temperature set unit provides a set signal indicating the desired temperature of the space.

In accordance with one aspect of the invention, a selection circuit is connected to receive both the set signal and a first reference signal. This selection circuit completes an operating circuit for the first thermostat when the value of the set signal is less than the value of the first reference signal, and completes an operating circuit for the second thermostat when the value of the set signal is greater than the value of the first reference signal. A comparator circuit is connected to receive both the set signal and the temperature signal from the energized thermostat, and to provide a regulating signal, related to the difference between the set signal and the temperature signal from the energized thermostat, to regulate associated heating and/or cooling equipment.

LII

In addition such an arrangement, when used with at least two compressors, includes a first time-delay circuit to prevent re-energization of the first compressor for a minimum time after the regulating signal decreases and the first cooling control stage indicates the first compressor should be taken off the line. A second time-delay circuit, coupled to the second cooling control stage which governs the second compressor, delays the energization of the second compressor when the regulating signal indicates a call for additional cooling, to prevent simultaneous energization of the compressors and a consequent high demand upon the electrical supply circuit.

THE DRAWINGS In the several figures of the drawings, like reference numerals identify like components, and in the drawings:

FIG. 1 is a block diagram, partly in schematic form, depicting the general arrangement of a portion of this invention;

FIG. 2 is a schematic diagram which sets out circuit details of components shown generally in FIG. 1; and

FIG. 3 is a schematic diagram which depicts the remainder of the control arrangement of the invention.

GENERAL SYSTEM DESCRIPTION FIG. 1 shows a general system layout of a portion of the control arrangement of this invention. Conventional equipment, such as a compressor and hot gas conduits utilized in regulating the cooling, and coils or other units for effecting the heating, are not depicted. Similarly the actual discharge and return air vents or conduits are not shown. Such components are well known and can be located in many environments, such as a railroad box car, truck, stationary enclosure, or any other space to be cooled and/or heated by this systern.

FIG. 1 shows a first thermostat 11 disposed to provide a first temperature signal which indicates the discharge air temperature. A second thermostat 10 is positioned to provide a second temperature signal which indicates the return air temperature. Both temperature signals do not appear on line 12 simultaneously, because selection circuit 13 completes an operating circuit for only one of the first or second thermostats, so that only one temperature indicating signal appears on line 12 at any one time. A temperature set unit 14, which can be a simple potentiometer or other device, provides a temperature set signal on line 15 and, over line 16, to selection circuit 13. The selection circuit also receives a first reference signal over line 17. When the value of the set signal is less than the value of the first reference signal, an operating circuit is completed over line 20 for first thermostat 11. When the value of the set signal exceeds the value of the first reference signal, an operating circuit is completed over line 18 for return air thermostat 10. This arrangement facilitates the establishment of a crossover point or reference temperature by the value of the reference signal passed over line 17 to selection circuit 13. For example this reference signal can be set to represent a temperature of 32F. Then if the temperature set unit 14 is adjusted to call for a temperature above 32, selection circuit l3 completes an energizing circuit over line 18 for return air thermostat l0, and allows this thermostat to do the controlling. When the temperature set unit 14 is adjusted to call for a temperature below the crossover point indicated by the signal on line 17, then the energizing signal is provided on line 20 so that the discharge air thermostat 11 does the controlling.

Comparator circuit 21 is connected to receive both the set signal, over lines and 22, and the temperature signal, over line 12, from whichever one of thermostats 10, 11 is then energized by selection circuit 13. The comparator stage provides an output or regulating signal on line 23 which is related to the difference between the set signal on line 22 and the temperature signal on line 12. This regulating signal is passed over line 24 and common line 25 to the cooling control stages, to be described below in connection with FIG. 3. The regulating signal is also passed from line 25 over resistors 26 and 27, and line 28 to the heating control circuitry, which will also be described in connection with FIG. 3.

In accordance with another aspect of this invention a heat override circuit 30 is provided, and has one input connection which receives a second reference signal over line 31. This second reference signalis usually of a different value then the level of the first reference signal supplied over line 17 to the selection circuit. The level of the second reference signal establishes a base against which the temperature set signal, also received by override circuit 30 from line 15, is compared to provide a lock-out signal on output conductor 32. For example, the amplitude of the reference signal on line 31 can be selected to represent a temperature of F. When temperature set unit 14 is adjusted to call for a temperature below 20, or below any other reference level established by the signal on line 31, an output signal is provided on line 32 and passed over diode 33 to conductor 28, to override the regulating signal from line 25. With this general perspective, a more detailed description of the invention will now be set out.

DETAILED DESCRIPTION OF THE INVENTION In the left hand portion of FIG. 2 a voltage divider arrangement 35 is shown connected between an energizing conductor 36 and a plane of reference potential, commonly designated ground. Resistors 37-43 are connected in series betweenconductor 36 and ground to establish different voltage levels or reference'signals, utilized to signify different temperature levels against which the actual temperature signals and the set point signal can be compared, and against which the regulating signal applied to the cooling and heating control stages can be referenced. Conductors 44-50, inclusive, are individually connected to different circuit points in the voltage divider 35 as shown to establish voltage levels, or reference signals, connoting different temperature levels. Another reference unit or voltage divider arrangement 51 includes resistors 52, 53 and 54 connected in series between conductors 47 and 50. Conductor 17 is connected to the junction of resistors 53 and 54, and is also coupled over resistor 55 to one input connection of an operational amplifier 56, connected as a voltage level detector in the thermostat selection circuit 13. Temperature set unit 14 is a potentiometer having its end connections coupled to conductors 47 and 50. The other input connection of op amp 56 receives the temperature set signal from the movable tap of potentiometer 14, over conductor 15, resistor 57 and conductor 16. The pin 4 connection of this op amp is grounded, and its 8 pin connection is coupled to conductor 36, or B+. This is also true (B+ on pin 8, and pin 4 grounded) for the other op amps 21 and 30 in FIG. 2, and for the control stage op amps 88 and in FIG. 3. All these units are connected as Schmitt trigger circuits. It will be understood that although different physical conductors are actually used for each stage, the same reference numeral 36 is employed to indicate the connection of the number 8 terminal to B+.

Selection circuit 13 also includes three NPN type transistors 58 60 and 61. The output connection of op amp 56 is coupled over a resistor 62 to input connection 3, and the output connection is also coupled to conductor 63. The base of transistor 58 is coupled to the common connection between resistors 64 and 65, which resistors are coupled in series between conductor 63 and ground. The emitter of transistor 58 is grounded and its collector is coupled to the common connection between resistor 66 and the anode of diode 67. The other end of resistor 66 is coupled to energizing conductor 36, and the cathode of diode 67 is coupled to the base of transistor 61. The base of transistor 60 is coupled over diode 68 and resistor 70 to conductor 63. The emitters of transistors 60, 61 are both connected to conductor 50, and the collectors of these two transistors are respectively coupled over conductors 18, 20 to the return and discharge thermostats 10, 11. The symbol T on these thermostats indicate a Thermistor is employed in the preferred embodiment of this circuit. The other connection of each Thermistor is coupled to conductor 12, and a resistor 71 is connected between conductors 12 and 47 to provide a return circuit for whichever one of Thermistors l0, 1 l is energized by circuit 13. I

In circuit 13, let it be assumed that the value of the first reference signal on line 17 represents a temperature of 32F. It is further assumed that the temperature called for by set unit 14 is below 32, and under these conditions it is desired to regulate the system with an output signal from discharge air thermostat 11. At this time the output signal from stage 56 is low, and there is no gate drive to transistor 60 to complete an operating circuit for Thermistor 10. However current flows from energizing conductor 36 over resistor 66 and diode 67 to the base of the transistor 61, driving this transistor on and completing an operating circuit'for the discharge air thermostat 11. Thus under these conditions the temperature signal from Thermistor 11 is passed over line 12 and resistor 72 to the 6 input connection of comparator circuit 21.

Assuming now that the temperature called for by unit 14 is changed, and is now above 32" or whatever reference level is represented by the signal on line 17, then the set signal on line 15 is effective over resistor 57 and conductor 16, to cause amplifier 56 to switch, and the output of this amplifier goes high. This provides gate driver over conductor 63, resistor 70 and diode 68 to the base of transistor 60, turning this transistor on to complete an energizing circuit for return air thermostat 10 over line 18. At the same time, as the signal on conductor 63 goes high, this provides a gate drive signal over the voltage divider circuit 64, 65 for transistor 58, which is driven on. As transistor 58 conducts it provides a virtual ground at the anode of diode 67, which robs transistor 61 of the gate drive previously received. Those skilled in the art will appreciate that the transition in control, from one thermostat to the other, can be achieved at any desired temperature by changing the level of the first reference signal provided on line 17.

Comparator circuit 21 receives the set signal over conductor 15, resistor 73 and conductor 22 at its 5 input terminal. This comparator circuit also receives the temperature-indicating signal, over line 12 and resistor 72, from the energized Thermistor. The output connection 7 of this stage is coupled over resistor 74 to a common connection 75. A regulating signal is provided from point 75 over conductor 24 to the associated control circuits to be described in connection with FIG. 3. A Zener Diode 76 is coupled between connection 75 and ground. Capacitor 77 is coupled in parallel with resistor 78, and this parallel circuit is coupled between terminal 75 and input connection 6 of comparator stage 21. The comparator stage operates in a well known manner to provide the regulating signal on conductor 24 and common line 25 as a function of the difference between the set signal received at input connection 5 and the temperature signal received at its other input connection 6.

At the bottom of FIG. 2 heat override stage 30 also comprises an op amp. Its 6 input connection is coupled over a resistor 80 to conductor 31, over which the second reference signal is received. The other input connection 5 is coupled over a resistor 81 to conductor 15,

over which the set signal is received. A capacitor 82 is coupled between the two input terminals, and a resistor 83 is coupled between the 5 input connection and the output connection 7, which output connection is also coupled to line 32. This stage 30 functions as the heat override circuit, to provide a lock-nut signal on line 32 to prevent operation of the heat equipment when the temperature called for by the set signal on line is below the value of the temperature represented by the second reference signal on line 31.

Considering now the circuitry depicted in FIG. 3, the regulating signal is received over line 24 and distributed over common conductor 25. In addition the signal from heat override stage 30 is received over conductor 32. Conductors 44-49, inclusive, are connected as already described to the difierent voltage points in the voltage divider-or reference arrangement 35, to pro-' vide different reference signals to the control stages in FIG. 3.

The op amps 85, 86, 87 and 88 are connected as cooling control stages to operate or switch in succession and provide actuating output signals to bring on additional cooling as the level of the regulating signal on common line 25 first exceeds that on line 48, then exceeds the level of the reference signal on line 46, next exceeds the reference signal on line 45, and finally is greater than the level of the reference signal established on conductor 44. The other op amp, 90, is a heating control stage used to provide an actuating output signal to associated heating equipment to bring on the heat. In that there is only one heat control stage, this will first be described.

A capacitor 91 is coupled between energizing conductor 36 and the common connection between resistors 26, 27 in the input circuit of op amp 90. The other input connection 3 of this stage is coupled over resistor 92 and conductor 49 to the voltage divider arrangement, to receive a reference signal as already described. A capacitor 93 is connected between the input terminals, and a resistor 94 is coupled between the output terminal 1 and the input connection 3 of this op amp.

Absent any override signa on line 32, the regulating signal on line 25 is applied to the input terminal 2, and the reference signal from conductor 49 is passed to the other input connection 3. When the level of the regulating signal exceeds that of the reference signal, op amp switches and provides a higher or positive output signal at connection 1, which is passed over resistor 95 to rapidly drive on NPN type transistor 96. Conduction of transistor 96 provides an output signal on conductor 97 to actuate the associated heating equipment. For example, this signal can operate a heat controlling relay to pass electrical current through heating coils and provide heat to the enclosed space regulated by the control system of this invention. In the circuit of transistor 96, diode 98 is connected between its collector and emitter as shown, and resistor 100 is coupled between its base and emitter. The emitter, one end of resistor 100, and the anode of diode 98 are connected to ground.

Considering now the first cooling control stage which includes op amp 85, the regulating signal on common line 25 is passed over resistor 101 to the 3 input connection of this stage. Capacitor 102 is coupled between the input connections. The reference signal supplied over line 48 is passed over resistor 103 to input connection 2. The output terminal 1 of this op amp is coupled over resistor 104 to input connection 3.

When the amplitude of the regulating signal on common line 25 passed over resistor 101 to terminal 3 exceeds the value of the reference signal present at input terminal 2, op amp 85 switches to provide a positive or high actuating output signal on conductor 105, which signal is passed over resistor 106 to the base of another NPN type transistor 107. This transistor is gated on to energize the first unit of cooling capacity. By way of example, this signal can complete the energizing circuit for the coil of a relay which puts the first of two compressors on the line, to initiate cooling of the controlled space. The circuit of transistor 107 includes a diode 108 and a resistor 110, connected in a manner similar to diode 98 and resistor 100 in the circuit of transistor 96.

' Considering now that the analog regulating signal on line 25 decreases as the space is cooled by the first compressor coming on the line, the signal level on common line 25 decreases and eventually falls below the level of the reference signal on line 48. At this time op amp 85 switches and goes low, so that compressor number one is deenergized at this time. If the analog signal on line 25 suddenly increases again, in a call for cooling, ordinarily the first compressor would immediately be switched on by operation of cooling control stage 85 and transistor 107. To avoid rapid cycling of this compressor, an important feature of this invention is a provision of an off time-delay circuit including a field-effect transistor (FET) 111. Considering the circuit of FET 111, energizing conductor 36 is coupled over a resistor 112 to the source of this transistor, and three high-ohm resistors 113, 114 and 115 are coupled in' series between conductor 36 and the upper plate of capacitor 116. A table of actual circuit components is set out at the end of the specification, but for the present it is sufficient to note that these three resistors provide a very high value of resistance between conductor 36 and capacitor 116. The lower plate of this capacitor is coupled over conductors 117 and 118 to the output side of op amp 85, which must go high and provide a positive signal to bring on the first compressor again. Another capacitor 120 is coupled between conductor 36 and a common circuit connection 121, with diode 122 coupled as shown between circuit point 121 and the source of PET 111. The drain of thistransistor is grounded. Another diode 123 also provides a reference connection between the heating circuit and common circuit point 121, which is also coupled to the 2 input connection of op amp 85. v

The first call for cooling, which provided the high or positive output signal from op amp 85, was applied over conductors 118 and 117 to the lower plate of capacitor 116. This signal allowed the capacitor 116 to be charged from conductor 36 over resistor 112, the source and gate of FET 111, and resistor 124 to the upper plate of capacitor 116. Thus virtually the entire B+ voltage, which was 24 volts positive on line 36 in a preferred embodiment, is developed across capacitor 116 as the first compressor is brought on the line. As the cooling is initially satisfied and op amp 85 switches to a low output signal, this in effect provides a ground or low level signal at the bottom plate of capacitor 1 16..

In effect this lower plate is now at minus B voltage, but the other side of capacitor 116 is returned through a very high resistance (113-115) to line 36. Thus it takes an appreciable time, determined by the high resistance in circuit with capacitor 116, for this negative voltage on lines 117 and 118 to decrease to the point where the positive switching of op amp 85 can override this voltage. In a preferred embodiment the timing circuit including FET l1 1 was set to provide approximately a 3- minute off interval of compressor number one. With this circuit the repeated turn-on of the first compressor in the cooling system is prevented, saving undue wear which might otherwise be caused as the regulating signal on common line 25 changed back and forth at a level very close to the reference signal on line 48.

It is noted that transistor 130 was gated on as the positive signal from op amp 85 was passed over line 118, resistor 132 and conductor 133 to the base of transistor 130. Another resistor 134 is coupled between the base and emitter of this transistor, and the diode 135 is con nected as shown between the collectorand emitter to protect transistor 130. Conduction of transistor 130 provides a signal on line 131 to energize the hot gas controlling relay, or any other suitable capacity control arrangement. This assumes a system in which the bypass of hot gas to the suction side of the compressor is utilized.

As the level of the regulating signal on line 25 continues to increase and eventually exceeds the level of the reference signal on line 46, op amp 86 will be switched to provide an output signal over line 125, resistor 126, and though resistor 127 to ground. Thus the voltage developed across resistor 127 provides a signal to the gate of transistor 128 which rapidly drives this transistor on and virtually grounds the base of transistor 130, to cut off this transistor and deenergize the hot gas controlling relay or whatever other hot gas control component may be connected to conductor 131. In the circuit of op amp 86, capacitor 136 is coupled between its input terminals. Input connection 6 receives a reference signal over resistor 137, and the other input connection is coupled over resistor 138 to common conductor 25, to receive the analog regulating signal. Resistor 140 is coupled between output connection 7 and input connection 5, and diode 141 is coupled between input connection 6 and conductor 142 to the on-time delay circuit for the second compressor.

As the amplitude of the regulating signal on line 25 further increases and exceeds the level of the signal on conductor 45, op amp 87 is switched to provide a high output signal at its terminal 7. This amplifier stage has a capacitor 143 connected between its input terminals, and input connection 6 receives the reference signal over resistor 144. The regulating signal on common line 25 is coupled over resistor 145 to the other input connection, which is also coupled over resistor 146 to output terminal 7. A diode 147 has its anode connected to the common connection between resistors 145 and 146 and capacitor 143, and its cathode coupled to conductor 118. As the output signalof terminal 7 of stage 87 goes high, this signal is extended over conductor 148 to the emitter of a'PNP type transistor 150. However this circuit includes another FET transistor 151 connected to provide a minimum on-delay, so that the second compressor cannot be energized by a signal of a step function type and an amplitude sufficient to bring on compressors one and two at the same time.

In this second time-delay circuit the source of unit 151 is coupled over resistor 152 to conductor 148, and the gate of the transistor is coupled to the common connection between a very high resistance 153 and the upper plate of a capacitor 154, the lower plate of which is grounded. The upper end of resistor 153 is coupled to conductor 148. The drain of PET 151 is coupled through a resistor 155 to ground. Another resistor 156 is coupled between conductor 148 and the common connection between resistor 155 and the drain of FET 151. The source of this transistor is coupled to the base of transistor 150, which has its collector coupled through a series circuit including resistors 157 and 158 to ground. Another NPN type transistor has its emitter grounded and its base coupled to the common connection between resistors 157 and 158. The collector of this transistor 160 is coupled over line 161 to provide another actuating output signal for bringing the second compressor on the line, as by completing a relayenergizing circuit for this compressor. A diode 162 is coupled between the base and emitter of transistor 160 forcircuit protection.

The resistances in the voltage divider circuit including FET 151 are selected so that the potential at the source of this unit is very close to the full B voltagefor this circuit as received over conductor 148. Resistor 156 is sized electrically to have a value considerably less than the resistance of resistor 152. Accordingly the voltage at the drain of PET 1511 is considerably lower than the voltage at its source. Thus even though the signal on line 48 is raised when cooling control stage 87 is switched, the voltage applied to the base of PNP type transistor 150 is not changed appreciably at this time, in that the voltage at the drain of FET 151 remains close to ground. It is only after capacitor 151 charges sufficiently to allow FET 151 to conduct, reducing the voltage at the base of transistor 150 and driving this transistor on to provide gate drive for transistor 160, that the signal is supplied over line 161 to bring the second compressor on the line. Accordingly this circuit functions as an inverse of the timing circuit including FET 111. This on-time delay circuit prevents simultaneous energization of the compressors to obviate a large drain on the electrical supply, which could blow a fuse or even damage equipment of the motorgenerator type which has a limited electrical capacity.

The last cooling stage including op amp 88 is operated when the level of the regulating signal on common line 25 passed over resistor 163 to the 3 input connection of this op amp exceeds the level of the reference signal passed over conductor 44 and resistor 164 to the 2 input connection. A capacitor 165 is coupled between the input connections of this stage, and a resistor 166 is coupled between output connection 1 and input connection 3. Thus as this stage switches and provides a high output signal on conductor 167, the actuating output signal is passed over resistor 168 to the common connection between resistors 126 and 127, to regulate the hot gas or other capacity control arrangement.

From this explanation it will be apparent that there is a sequential operation of the cooling control stages 85-88 and the associated cooling equipment as the amplitude of the analog regulating signal on common line 25 gradually increases when the system calls for additional cooling. From the set point established by unit 14, when the temperature in the control space rises 75, op amp 85 switches to bring the first compressor on the line and, through transistor 130, to bring the hot gas on. With an additional half a degree rise to one degree above the set point, op amp 86 is switched high to drive transistor 128 on and rob the drive from transistor 130, turning the hot gas off. With an additional half a degree temperature rise to a value 1.5 degrees F. above the set point, op amp 87 is switched on to initiate the timing interval so that after FET 151 conducts, transistors 150 and 160 are driven on to bring the second compressor on the line. As soon as op amp 87 conducts, a signal is extended from output pin 7 over resistor 146, diode 147, and resistor 132 to energize transistor 130 and bring the hot gas relay on. With a further increase in temperature of half a degree, or a total of two degrees F. above the set point, op amp 88 is switched to provide a signal over line 167 and resistor 168 to drive on transistor 128, turn off transistor 130, and shut off the hot gas. With a reversal of the temperature change in the controlled space, the same functions occur in the opposite sequence. The value of the reference signal on line 49 is established so that as op amp 90 is switched, and the heat brought on, when the temperature in the controlled space is one degree F. below that established by the temperature set unit 14.

To enable those skilled in the art to practice the invention, a table of circuit component identifications and values is set out below. These are given solely to enable those skilled in the art to practice the invention with a minimum of experimentation, and in no way rep resent a constraint upon the concept or implementation of the invention. The circuit described and illustrated was operated with a d-c potential of 24 volts positive on conductor 36 with respect to ground. The integrated circuits shown, including the units 21, 30, 56, 85-88 and 90, were all of the Signetics type N555 8V. The transistors, other than those identified below, were 2N34l6 and the diodes were BAX 1 8. In general the resistors were one-half watt, percent, unless otherwise specified.

While only a particular embodiment of the invention has been described and illustrated, it is manifest that various modifications and alterations may be made therein. It is therefore the intention in the appended claims to cover all such modifications and alterations as may fall within the true spirit and scope of the invention.

What is claimed is:

l. A control arrangement for regulating operation of a cooling system which discharges air into a space and receives return air from the space, which control arrangement comprises:

a first thermostat disposed to provide a temperature signal indicative of the discharge air temperature;

a second thermostat, disposed to provide a second temperature signal indicative of the return air temperature;

a temperature set unit, adjustable to provide a set signal indicating the desired temperature of the space;

a selection circuit, connected to receive both the set signal and a first reference signal, which selection circuit completes an operating circuit for the first thermostat when the value of the set signal is less than the value of the first reference signal, and completes an operating circuit for the second thermostat when the value of the set signal is greater than the value of the first reference signal; and

a comparator circuit, connected to receive both the set signal and the temperature signal from the energized one of the thermostats, and to provide a regulating signal, related to the difference between the set signal and the temperature signal from the energized thermostat, to regulate associated equipment.

2. A control arrangement as claimed in claim 1, and further comprising a heat override circuit, connected to receive both the set signal and a second reference signal, operative to provide a lock-out signal to prevent operation of associated heating equipment when the temperature called for by the set signalis below the value of the temperature represented by the second reference signal.

3. A control arrangement for regulating operation of a cooling system which discharges air into a space and receives return air from the space, which control arrangement comprises:

a first Thermistor disposed to provide a first tempe rature signal indicative of the discharge air temperature;

a second Thermistor, disposedto provide a second temperature signal indicative of the return air temperature;

a temperature set potentiometer, adjustable to provide a set signal indicating the desired temperature of the space;

a selection circuit, including a voltage level detector connected to receive both the set signal and a first reference signal, a first transistor connected to complete an operating circuit for the first Thermistor when the value of the set signal is less than the value of the first reference signal, and a second transistor connected to complete an operating circuit for the second Thermistor when the value of the set signal is greater than the value of the first reference signal; and

a comparator circuit, connected to receive both the set signal and the temperature signal from the energized one of the Thermistors, and to provide a regulating signal, related to the difference between the set signal and the temperature signal from the energized Thermistor, to regulate associated equipment.

4. A control arrangement for regulating operation of a cooling system which discharges air into a space and receives return air from the same space, including first and second thermostats disposed to provide temperature signals indicating the discharge and return air temperature, a temperature set unit providing a set signal to indicate the desired space temperature, a selection circuit completing an operating circuitto only one of the thermostats as a function of the set signal and a first reference signal, a comparator circuit receiving the set signal and the temperature signal from the energized thermostat to provide on a common line a regulating signal which is a function of the difference between the set signal and the actual temperature signal from the energized thermostat, and at least two cooling control stages, each having an input portion coupled to the common line, and each having an output circuit individually coupled to different cooling components, to bring on one of the different cooling components at different amplitude levels of the regulating signal on the common line.

5. A control arrangement for regulating operation of a cooling system by comparing a temperature set signal with an actual temperature signal and providing a regulating signal, related to the difference between the set signal and the actual temperature signal to regulate cooling equipment including at least two compressors through a plurality of cooling control stages, which control arrangement comprises:

a common line connected to receive the regulating signal;

a first cooling control stage, having a first input connection coupled to the common line and a second input connection connected to receive a first reference signal, to provide an actuating output signal when the regulating signal exceeds the first reference signal and bring the first compressor on the line;

a second cooling control stage, having a first input connection coupled to the common line and a second input connection coupled to receive a second reference signal, to provide an actuating output signal when the regulating signal exceeds the second reference signal and bring the second compressor on the line; and

a first time-delay circuit, coupled to said first cooling control stage, effective to maintain said first controlling stage in the off condition for a predetermined minimum time interval after the regulating signal on the common line has decreased below the value of the first reference signal, to avoid excess cycling of the first compressor.

6. A control arrangement as claimed in claim 5, and

further comprising a second time-delay circuit, coupled to said second cooling control stage, effective to prevent said second cooling control stage from providing